Convert date to float for linear regression on Pandas data frame

Question

It seems that for OLS linear regression to work well in Pandas, the arguments must be floats. I'm starting with a csv (called "gameAct.csv") of the form:

date, city, players, sales

2014-04-28,London,111,1091.28

2014-04-29,London,100,1100.44

2014-04-28,Paris,87,1001.33

...

I want to perform linear regression of how sales depend on date (as time moves forward, how do sales move?). The problem with my code below seems to be with dates not being float values. I would appreciate help on how to resolve this indexing problem in Pandas.

My current (non-working, but compiling code):

import pandas as pd

from pandas import DataFrame, Series

import statsmodels.formula.api as sm

df = pd.read_csv('gameAct.csv')

df.columns = ['date', 'city', 'players', 'sales']

city_data = df[df['city'] == 'London']

result = sm.ols(formula = 'sales ~ date', data = city_data).fit()

As I vary the city value, I get R^2 = 1 results, which is wrong. I have also attempted index_col = 0, parse_dates == True' in defining the dataframe df, but without success.

I suspect there is a better way to read in such csv files to perform basic regression over dates, and also for more general time series analysis. Help, examples, and resources are appreciated!

Note, with the above code, if I convert the dates index (for a given city) to an array, the values in this array are of the form:

'\xef\xbb\xbf2014-04-28'

How does one produce an AIC analysis over all of the non-sales parameters? (e.g. the result might be that sales depend most linearly on date and city).

Answer 1

For this kind of regression, I usually convert the dates or timestamps to an integer number of days since the start of the data.

This does the trick nicely:

df = pd.read_csv('test.csv')
df['date'] = pd.to_datetime(df['date'])    
df['date_delta'] = (df['date'] - df['date'].min())  / np.timedelta64(1,'D')
city_data = df[df['city'] == 'London']
result = sm.ols(formula = 'sales ~ date_delta', data = city_data).fit()

The advantage of this method is that you're sure of the units involved in the regression (days), whereas an automatic conversion may implicitly use other units, creating confusing coefficients in your linear model. It also allows you to combine data from multiple sales campaigns that started at different times into your regression (say you're interested in effectiveness of a campaign as a function of days into the campaign). You could also pick Jan 1st as your 0 if you're interested in measuring the day of year trend. Picking your own 0 date puts you in control of all that.

There's also evidence that statsmodels supports timeseries from pandas. You may be able to apply this to linear models as well: http://statsmodels.sourceforge.net/stable/examples/generated/ex_dates.html

Also, a quick note: You should be able to read column names directly out of the csv automatically as in the sample code I posted. In your example I see there are spaces between the commas in the first line of the csv file, resulting in column names like ' date'. Remove the spaces and automatic csv header reading should just work.